1. Field of the Invention
The present invention generally relates to voltage-controlled oscillators, and more specifically, the invention relates to architectures for maintaining constant voltage controlled oscillator gain. Even more specifically, the preferred embodiment of the invention relates to such an architecture that is very well suited for use in phase-locked loops in order to make loop dynamics uniform across the tuning range of the voltage controlled oscillator.
2. Background Art
Phase-locked loops (PLLs) are commonly used to generate high-frequency signals in a wide variety of electronic circuit applications. Many important PLL parameters (jitter, settling time) depend on the bandwidth of the PLL. Loop bandwidth in turn depends on, among other parameters, the gain of the oscillator in the PLL. In the case of a voltage-controlled oscillator (VCO), the gain, which is the incremental slope of its output frequency vs. tuning voltage characteristic, has the units Hz/V. For many VCO types (e.g., LC, ring VCOs of the current starved type, ring VCOs of the delay interpolating type, or ring VCOs of a hybrid type), their output frequency vs. tuning voltage characteristic is nonlinear; giving rise to a VCO gain that varies as a function of tuning voltage. For many VCOs, the input voltage is converted to a current (by a voltage to current converter (V-I converter)), which in turn is applied to a circuit and which can be viewed as a current controlled oscillator (ICO). Non-linearity in the overall VCO tuning characteristic can stem from non-linearity in the ICO's output frequency vs. tuning current characteristic, or from non-linearity in the V-I converter's current vs. voltage characteristic. Solutions exist to create a VCO with a linear characteristic by introducing a non-linearity into the V-I converter that compensates for the inherent non-linearity of the ICO.
Single port VCOs do not always enable adequate PLL performance due to tradeoffs associated with VCO gain. High VCO gain helps enable a wide tuning range, while low VCO gain helps reduce noise sensitivity. Architectures have been proposed that split the VCO control path into two (or more) paths. In the case of a two-path control loop, one path has lower gain and high bandwidth, while the other path has high gain and lower bandwidth. This splitting reduces the VCO's noise sensitivity by allowing for substantial filtering on the high VCO gain path, however, the VCO will still have a nonlinear tuning characteristic.